Configurable radiative loss function

[wtbarnes]

Currently, the radiative loss function is specified as a power-law according to Eq. 3 of Klimchuk et al. Instead of just hardcoding this, we should allow the user to specify a different loss function if they'd like, with the original loss function as the default option.

[jwreep]

The time-variable abundance has four different options, but none are customizable. It wouldn't be hard to add a "custom" option to the configuration file (say custom abundances), but how would we generate either the look-up table or power-law? Would this be a full calculation like in HYDRAD? That might slow the code noticeably.

[wtbarnes]

I was thinking of just the ability to provide a look-up table, probably as a path to a file.